If bouldering indoors, how do you know if a problem requires a sit-start or not?

1) if this is a competition, check the rules, look for anything written on the stating hold tape, and ask the route-setters if you are not sure, because you don't want to blow your first attempt.

2) if this is just bouldering, then who the heck cares? Do it how you want to/how it seems obvious to you. You need to start with hands on the starting holds, and feet off the ground. If you can do so without sitting down on a starting hold that is 2 feet off the ground, then you did it. If someone watches you do it, and comments that the route is a sit-start, repeat it again with a sit start. Or not.

My Gym marks sit starts by marking the start holds with downward pointing arrows made from the route tape. Upward facing arrows on the start hold are for normal starts. Just ask the employees at your gym to see what they say though!